For many years the standard practice for calibrating shearographic/holographic tire testing machines has been ASTM F1364-92. This test method describes the construction and use of a calibration device for demonstrating the anomaly detection capability of an interferometric laser imaging non-destructive tire inspection system. A typical shearographic fringe pattern resulting from such testing technique is shown in FIG. 1.
As has been described in U.S. Pat. No. 6,433,874 the technique of shearing interferometry, or shearography involves the interference of two laterally-displaced images of the same object to form an interference image. Conventional shearographic methods require that a first interference image (or baseline image) be taken while the object is in an unstressed or first stressed condition, and another interference image be taken while the object is in a second stressed condition. Comparison of those two interference images (preferably by methods of image subtraction) reveals information about the strain concentrations and hence the integrity of the object in a single image called a shearogram. FIG. 1 shows an image which is the direct result of two laterally-displaced images being obtained by an interference technique. The images that are obtained are not to the scale of the anomaly in the tire. In addition very minute anomalies, those in the order of 1.7 mm are not readily ascertainable from a shearogram such as that shown in FIG. 1.
While some systems for shearography have an output such as the display of computerized systems of the '874 patent, many of the systems utilized are such that they have an output of highly sensitive film which is extremely costly and requires a special viewing device. Further, film based shearographic tire testing machines (and individual tire tests) are typically very expensive therefore a limited number are utilized in the industry within individual plants.
The electronic shearography of the '874 patent is based upon the shearography described in U.S. Pat. No. 4,887,899 which describes an apparatus which produces an interference image by passing light, reflected from the test object, through birefringent material and a polarizer. The birefringent material splits a light ray into two rays and polarizing it makes it possible for light rays reflected from a pair of points to interfere with each other. Thus each point on the object generates two rays and the result is an interference image formed by the optical interference of two laterally displaced images of the same object.
There is a need for a tire testing technique that facilitates direct measurement of anomalies in order for the anomalies to be quantified.
There is a need to obtain an output from tire testing equipment which does not rely upon an interferogram or proprietary optics, thereby making the output of such testing technique accurately describe and identify the anomaly.
There is a need for a tire testing technique and apparatus which permits the utilization of coherent light and the reflection of such light to be captured by inexpensive equipment and displayed using commonly available computerized systems.
It is accordingly an object of the invention to provide a new tire testing technique and apparatus which facilitates direct measurements, and a quantitative analysis or scalability of the anomaly in the tire.
It is an object of the present invention to provide a new tire testing technique and apparatus which does not rely upon an interferogram to accurately describe and identify the anomaly.
It is an object of the present invention to provide a new tire testing technique and apparatus which permits the utilization of coherent light and reflection of such light to be captured by inexpensive equipment and displayed using commonly available computerized systems which does not utilize laterally-displaced images but a single image of the anomaly as output.